Regarding the catalyst for producing acrylic acid through vapor-phase contact oxidation of acrolein, many composite oxide catalysts have heretofore been known as described in Patent Reference 1, Patent Reference 2, etc. Some of those catalysts have already been industrialized and used in production of acrylic acid, but the productivity thereof is not always satisfactory; and with the recent growth in demand for acrylic acid, more highly-productive catalysts have become desired.
In industrially producing acrylic acid with the catalyst, there is a demand for further increasing the productivity of acrylic acid per the catalyst unit volume and for further prolonging the catalyst life; however, when the flow rate of acrolein as material source is increased for the purpose of increasing the productivity of acrylic acid, then the reaction heat may increase and the hot spot temperature may be thereby raised owing to the accumulation of heat in the catalyst layer, therefore providing a possibility of not only resulting in yield reduction owing to excessive oxidation and in acceleration of catalyst degradation owing to thermal load but also causing runaway reaction.
Some proposals have been made for the means of depressing the temperature of hot spot areas. For example, there have been proposed a method of diluting the catalyst layer on the gas inlet port side with an inert substance (Patent Reference 3); a method of sequentially increasing the catalytically-active substance-supporting ratio (ratio by weight of active substance per catalyst) toward the gas outlet port side from the gas inlet port side (Patent Reference 4); a method of sequentially reducing the size of the catalyst toward the gas outlet port side from the gas inlet port side (Patent Reference 5); a method of lowering the catalyst activity on the gas inlet port side through addition of an alkali metal thereto (Patent Reference 6); a method of varying the catalyst ingredients and/or the amount thereof and charging the catalysts having a different activity in the system in such a manner that the activity of the catalyst could increase toward the gas outlet port side from the gas inlet port side (Patent Reference 7), etc.